216 ELEMENTARY LESSONS ON [CHAP, iv, 



Sp units of electricity ; hence the total force of attraction 

 on the disc will be F = 2?r/o x Sp. 



From this we gather that, if the force F remain the 

 same throughout the experiments, the difference of po- 

 tentials between the discs will be simply proportional to 

 the distance between them when the disc is in level 



/o -p 



equilibrium. And the quantity / ~^- may be deter- 



mined once for all as a "constant" of the instrument. 



In the more elaborate forms of the instrument, such 

 as the " absolute electrometer," and the "portable 

 electrometer," the disc and guard -plate are covered 

 with a metallic cage, and are together placed in com- 

 munication with a condenser to keep them at a known 

 potential. This obviates having to make measurements 

 with zero readings, for the differences of potential will 

 now-'be proportional to differences of micrometer readings^ 



The condenser is provided in these instruments with 

 a gauge, itself an attracted-disc, to indicate when it is 

 charged to the right potential, and with a replenisher to 

 increase or decrease the charge, the replenisher being 

 a little convection-induction machine (see Art. 45). 



262. The Quadrant Electrometer. The Quad- 

 rant Electrometer of Sir W. Thomson is an example of 

 a different class of electrometers, in which use is made 

 of an auxiliary charge of electricity previously imparted 

 to the needle of the instrument. The needle, which con- 



